Abstract:

The present invention relates the manufacture of metal powders, non-oxidic
ceramic powders and reduced metal oxide powders using an improved flame
spray pyrolysis ("FSP") process. The invention further relates to an
apparatus specifically adapted to said process, to powders/naoncomposites
obtained by said process and to the use of said powders/nanocompsites.

Claims:

1. A method for the production of powders selected from the group
consisting of metal powders, non-oxidic ceramic powders and reduced metal
oxide powders as well as mixtures/alloys thereof,wherein said method
comprises flame spray pyrolysis (FSP) of a combustible precursor
solution, andwherein said method being performed in an atmosphere with an
O2 concentration below 100 ppm (volume/volume) in the flame off-gas
(measured above the flame where the flame off gas has cooled to below
200.degree. C.) and wherein said method being performed using an
oxygen-containing gas as oxidizing agent.

14. The method of claim 1 wherein the powders produced are selected from
the group of metals having a standard potential between +0.52 eV and
-0.80 eV and alloys containing a metal having a standard potential
between +0.52 eV and -0.80 eV.

15. The method of claim 14 wherein the powders produced are selected from
the group of metals consisting of a standard potential between +0.52 eV
and -0.42 eV and alloys containing a metal with a standard potential
between +0.52 eV and -0.42 eV.

16. The method of any one of claim 1, wherein the particles produced are
non-oxidic ceramic particles, selected from the group of transition metal
carbides, ˜nitrides, ˜sulfides, ˜silicides,
˜selenides, ˜tellurides, ˜borides.

17. The method of claim 1 wherein H2 and/or CO are separated from the
off gas and purified.

18. The method according to claim 1, wherein one or more metal comprising
compounds selected from the group of Ag, Fe, Co, Cu, Mo, Bi, W are used
and wherein the combustible solvent is selected from aliphatic, cyclic or
heterocyclic compounds containing no aromatic systems or long aliphatic
chains (e.g. >C8) and wherein the fuel-oxygen equivalence ratio is
between 1.0 and 2.0.

19. A Flame Spray Pyrolysis (FSP) apparatus wherein the flame is shielded
from environmental air by means of a porous tube and wherein said porous
tube optionally allows radial application of gases.

20. (canceled)

21. The FSP apparatus of claim 19 wherein said porous tube is a sinter
metal tube or a porous graphite tube or a porous silicium-carbide tube.

22. The FSP apparatus of claim 19, wherein said porous tube consists of a
porous inner tube, and distant there from a tight outer tube and
optionally devices allowing for application of gas through the porous
inner tube.

23. The FSP apparatus of claim 19, wherein the flame and the collector are
located in an airtight box filled with inert and/or reducing gas.

24. The FSP apparatus of claim 19 wherein said FSP apparatus is connected
to a gas/steam turbine and wherein the off-gasses of said FSP apparatus
are used to operate said gas/steam turbine.

26. Powder obtained by the method of claim 1, wherein the maximal
geometric standard deviation σg of the particle size is
smaller than 1.6.

27. Powder obtained by the method of claim 1, wherein said powder is
selected from the group of metal powders and wherein said powder gains at
least 70% of the weight which corresponds to a full oxidation of the pure
metal to the corresponding thermodynamically stable oxide upon a
treatment consisting of a 1 hour oxidation in ambient air at 800.degree.
C.

28. Powder obtained by the method of claim 1 having a pressure and
temperature dependent conductivity wherein said temperature dependent
conductivity shows a negative coefficient of at least 3500 K linearized
between 30 and 100.degree. C.

29. Powder obtained by the method of claim 1, wherein said powders are
made of metals or alloys where at least one constituent is selected from
the group comprising Co, Cu, Fe, Ni and wherein said powders--after
compaction at above 50 MPa and heating in an inert or reducing atmosphere
to a maximum temperature of 60% of the melting temperature of the
metal--show a Vickers hardness of at least 2 times of the Vickers
hardness of the corresponding bulk coarse grained (>10 microns)
material.

30. A nanocomposite essentially consisting of ceramic particles and metal
particles, wherein the number averaged primary particle size of said
ceramic particles and metal particles are both below 300 nm.

32. A nanocomposite according to claim 30, wherein the hardness--as
measured by the Vickers method--is at least twice the Vickers hardness of
the pure, coarse-grained, bulk metal.

33. A nanocomposite according to claim 30, wherein essentially all ceramic
particles are in contact with at least one metallic particle as evaluated
from transmission electron microscopy.

Description:

FIELD OF THE INVENTION

[0001]The present invention relates the manufacture of metal powders,
non-oxidic ceramic powders and reduced metal oxide powders using an
improved flame spray pyrolysis ("FSP") process. The invention further
relates to an apparatus specifically adapted to said process, to
powders/naoncomposites obtained by said process and to the use of said
powders/nanocompsites.

BACKGROUND ART

[0002]Flame spray pyrolysis (WO2005/103900, EP1378489) represents a cost
efficient process for the manufacturing of high quality metal oxide
nano-powders. The resulting powders show a very high surface area
together with high material purity. The process further gives the
possibility of producing mixed metal oxides with a high homogeneity both
in chemical composition and powder particle size. The powders are mostly
un-agglomerated and of very narrow size distribution. It was recently
shown (Loher et al. 2005, Huber et al. 2005, Grass and Stark 2005) that
the process is also capable of producing salts such as calcium-phosphates
and fluorides.

[0003]Metal, non-oxidic ceramic and reduced metal oxide nano-powders which
are currently not available from flame spray pyrolysis are of large
industrial interest: Besides giving very colourful materials for use in
pigments (Bahador 1995), reduced metal oxides exhibit semi-conducting
properties (Lou et al. 2005) and a high ion conductivity of interest for
electronic applications and solid state fuel cells. Non-oxidic ceramics
such as tungsten-carbide, cobalt-nitride and many others display
excellent mechanical properties (GB696589) such as very high hardness and
temperature resistance making them of interest for high duty applications
such as cutting tools and protective coatings. Nano-sized metal powders
such as iron, steel, copper, cobalt and others are of interest for powder
metallurgy. Further, these materials exhibit size dependent
characteristics (Modrow et al. 2005) such as enhanced electronic,
magnetic (Kodama 1999) or mechanical properties giving them manifold
applications in the electronic and machining industry. All three groups
of materials have applications as reactive surfaces, as ceramics,
building materials and in heterogeneous catalysis, especially when the
particles are of small size exhibiting large surface areas. Two selected
examples of catalysts of interest are tungsten-carbide for platinum-like
catalysis such as hydrogenation (Levy and Boudart 1973) and metal
nitrides as well as alloys of metal nitrides for hydrodenitrogenation
(Milad et al. 1998; Wang et al. 2005). Further applications include low
melting alloys for interconnects in electronics (Li et al. 2005).

[0004]Currently metal powders and alloys are produced by a series of
different processes depending on the necessary product size and purity.
For large metal particles (above 1 micrometer) atomization of liquid
metal using a nozzle, disk or cup (see e.g. US 2005/009789 and references
therein) is used as an efficient low cost method. Particle size is
strongly limited by the smallest liquid droplet which can be formed.
Smaller particles can be formed by alloy leaching (see e.g. WO
2004/000491). This process is limited to only a few metals and their
alloys, results in large amounts of liquid waste and leads to strongly
agglomerated particles which have to be de-agglomerated (e.g. by
milling). Spherical, un-agglomerated and monodisperse metal
nano-particles can also be obtained by wet-phase chemistry (such as
Nicolais 2005). Besides the large liquid waste produced by these
processes, the application of the produced particles is limited to the
liquid phase as it is difficult to dry the powders completely without
leaving surfactants and solvents contaminating the residual product. High
temperature electronic processes, such as lasers (Dez et al. 2002) and
plasma reactors (e.g. U.S. Pat. No. 5,486,675, GB 2 365 876, DE 39 37
740) are used for the fabrication of nanosized metal powders. Due to the
high necessary temperatures (several 1000 K), the high cost of electrical
energy and low efficiency, these processes remain relatively expensive
and complex. A further method for the synthesis for metal nano particles
is vapor flow condensation (see Wegner et al. 2002 and references
therein). This process however is limited to metals with low vaporisation
temperature. Several pyrolysis processes in hot tubes have been reported
in the academic literature (Eroglu et al. 1996, Knipping et al. 2004) but
all are limited to low production rates. The major disadvantage of these
processes is the diffusion and thermophoresis of particles to the tube
wall and therefore lowering the process yield making up-scaling
difficult. All state of the art processes give particles with a broad
size distribution that is undesirable in most applications.

[0005]It is therefore of great industrial interest to have a production
method which best combines cost efficiency and versatility.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]The invention will be better understood and objects other than those
set forth above will become apparent when consideration is given to the
following detailed description thereof. Such description makes reference
to the annexed drawings, wherein:

[0007]FIG. 1 shows the accessible materials of this invention in
comparison to state of the art materials in terms of process complexity
and standard formation potential as defined in Riedel (1999).

[0008]FIG. 2 shows a possible setup as used for example 1-12. A flame
spray nozzle 21 as described by Madler et al. (2002) was placed in a
glove box 24. The off gas was freed from the powder by a glass-fiber
filter 22 and piped into a series of 2 columns 23 filled with zeolite 4A
and 13X, respectively, for the removal of water and carbon dioxide.

[0013]FIG. 6 shows a transmission electron micrograph of the product
produced in example 4. Unagglomerated, spherical cobalt particles with a
narrow particle size distribution are visible. Some of the particles show
the formation of twins or other distortions of the crystal lattice.

[0014]FIG. 7 shows the magnetization curve of the powder produced in
example 4 as measured on a superconducting quantum interface device. The
inset shows a magnification from -1000 to 1000 Oe showing the hysteresis.

[0015]FIG. 8 shows XRD pattern of powders produced in examples 5 (A) and 6
(B). Reference peaks are shown as stars (alpha-iron), as triangles
(gamma-iron) as well as squares (iron(II) oxide; Wustite).

[0016]FIG. 9 shows a XRD pattern of the product prepared by example 7. The
reference peaks for tungsten is shown as star, the reference peaks for
tungsten(VI) oxide are shown as squares.

[0017]FIG. 10 shows a XRD pattern of example 9 together with reference
peaks for copper (squares).

[0018]FIG. 11 shows 2 transmission electron micrographs of the powder
produced in example 8. Small copper particles are visible which are
embedded in a carbon matrix.

[0019]FIG. 12 shows 2 transmission electron micrographs of the powder of
example 9. Picture A again shows a small particle size distribution and
picture B shows the carbon coating of the copper.

[0020]FIG. 13 shows the stability of the powder prepared in example 9 in
nitric acid. The mass loss was identified by copper titration by EDTA
against Murexide.

[0021]FIG. 14 shows the temperature (left) and pressure (right) dependence
of the electronic resistively of a pill prepared from powder synthesized
in example 9.

[0022]FIG. 15 shows the XRD pattern of the powder produced in example 11
together with the reference peaks for bismuth as stars.

[0023]FIG. 16 shows a scanning electron micrograph of the as produced
powder of example 11. The particles of 30 nm diameter are strongly
agglomerated and very homogeneous in size.

[0024]FIG. 17 shows an XRD pattern of the produced powder of examples 12
(B) and 6 (A). Reference peaks for alpha-iron (square), gamma-iron
(triangle) and wairauite (CoFe, star) are also shown.

[0025]FIG. 18 shows the dependence of the maximum sintering temperature on
the Vickers hardness of nanocrystalline cobalt compacts produced
following ex. 13 (pressed at 370 MPa). Reference values for bulk cobalt
and stellite alloy (Stellite 6: Co-28Cr-4W-1.1C) are shown.

[0026]FIG. 19 shows a transmission electron micrograph of a cobalt sample
pressed at 370 MPa from nano-cobalt powder (example 13) and sintered at
900° C. in a Hydrargon® atmosphere showing nanocrystalline
grains of fcc-cobalt with a high density of nanovoids (indicated by
arrows in a) and twin defects (indicated by arrows in b) stabilizing the
material and preventing grain growth.

[0029]FIG. 22 shows a photograph of a bismuth/ceria composite (15 vol %
ceria, example 15) compacted in an uni-axial press at 370 MPa in an
electrical current showing good conductivity (right). Scanning electron
micrograph (left) of the pill surface showing large bismuth grains
separated by much smaller ceria particles

DISCLOSURE OF THE INVENTION

[0030]Hence, it is a first object of the invention to provide an improved
method for the production of powders different from metal oxides, in
particular nano particles of metals, of non-oxidic ceramics, of reduced
metal-oxides and of its mixtures/alloys.

[0031]A second object of the present invention is to provide an improved
device/apparatus for the manufacture of said powders.

[0032]Another object of the present invention are powders obtainable by a
method of the present invention.

[0033]Yet another object of the present invention is the use of such
powders.

[0034]These objectives are achieved by a method as defined in claim 1 and
an apparatus as defined in claim 19. Further aspects of the invention are
disclosed in the specification and independent claims, preferred
embodiments are disclosed in the specification and the dependent claims.

[0035]Unless otherwise specified, the following definitions shall apply in
this specification.

[0036]The term "reduced metal oxides" refers to metal oxides wherein the
metal is in a lower oxidation state than the thermodynamic most stable
oxidation state at room temperature.

[0037]The "protecting gas" is known in the field and refers to inert gases
such as nitrogen, argon or helium and mixtures thereof.

[0038]The "Reducing gases" are known in the field and include hydrogen and
methane and CO. Preferably, such reducing gases are directly applied to
the flame.

[0039]The "fuel-oxygen equivalence ratio" is defined as the ratio of the
actual fuel/air ratio to the stoichiometric fuel/air ratio, calculated
from the gases leaving the flame nozzle, neglecting the gases present in
the atmosphere surrounding the flame and the gases fed towards the flame
and off gas after ignition e.g. through the sinter metal tube. The
stoichiometric fuel/air ratio concerns the full combustion of the fuel to
CO2 and H2O.

[0040]The "standard potential" is the potential measured electrochemically
against a standard hydrogen electrode at a concentration of 1 Molar for
all dissolved components, a pressure of 1 atmosphere and a temperature of
25° C. (c.f. Riedel 1999)

[0041]The term "powders" as used in this specification refers to finely
divided granular material, e.g. obtained from an FSP process. Typically,
the individual granules of a powder have similar properties.

[0042]The present invention will be described in more detail below. It is
understood that the various aspects, embodiments, preferences and ranges
as provided/disclosed in this specification may be combined at will.
Further, depending of the specific embodiment, selected definitions,
embodiments or ranges may not apply.

[0043]In a first aspect, the invention relates to a method for the
production of powders selected from the group consisting of metal
powders, non-oxidic ceramic powders and reduced metal oxide powders as
well as mixtures and alloys thereof, wherein said method comprises flame
spray pyrolysis (FSP) of a combustible precursor solution, and wherein
said method being performed in an atmosphere with an O2
concentration below 1000 ppm (volume/volume) measured in the off-gas and
wherein said method being performed using an oxygen-containing gas as
oxidizing agent.

[0044]To better localize the present invention with regard to known
methods accounting for process complexity and producible metal of given
standard potential, an illustration is given in FIG. 1. In general, it is
sufficient to shield the flame and preferably also the production path up
to the particle collection from ambient air to achieve the desired
O2 concentration. In a preferred embodiment, however, the flame and
the production path are incorporated into a porous tube, e.g. a sintered
tube that preferably is incorporated in a larger tight tube. In the case
of two tubes, an additional shield may be employed. This configuration
allows forming a gas film on the tube surface thereby stabilizing the
porous tube and also cooling the flame such enabling to control the flame
temperature. In order to control particle deposition, the tube may be
cooled in the proximity of the collector.

[0045]The method of the present invention makes to some extent use of
flame spray pyrolysis (FSP) methods previously developed for oxides. Due
to amendments (as described herein) relating to the apparatus and to the
process, new materials are obtainable.

[0046]In one embodiment, inert gases such as N2, CO2 or noble
gases are used as gas shield and applied to reduce the oxygen content in
the flame and adjacent to the flame. Optionally, the inert gases are
recycled after H2O and CO2 removal (by e.g. zeolites) and
H2 recovery (by e.g. loading H2-carrier materials).

[0047]In an alternative embodiment, reactive gases such as H2 and
hydrocarbons are directly applied to the flame such as the "combustion
gas" O2 or N2O.

[0049]In general, the molar hydrogen content in the off gas is at least
double, more preferably at least 5 times, most preferably at least 20
times the molar metal content in the off gas.

[0050]In an advantageous embodiment, the oxygen-containing gas is air. In
a further advantageous embodiment, the oxygen-containing gas is
commercial available oxygen (e.g. technical grade oxygen). In a further
advantageous embodiment, the oxygen-containing gas is a gaseous compound
containing oxygen (e.g. N2O).

[0051]Combustible precursor solutions" suitable for FSP methods as
described herein are known and are described e.g. for the manufacture of
metal oxides using FSP. Such precursor solutions comprise one or more
metal comprising compounds ("precursor") and optionally one or more
combustible solvents. The precursors in general are metal comprising
compounds which are soluble in combustible solvents. The precursors
include organometallic compounds, metal salts and metal complexes such as
2-ethylhexanoates, carboxylates, halogenides, cyclopentadiens and the
like. The precursors used are preferably combustable organometallic
compounds or oily solutions of metals as well as oil soluble metal
complexes and salts and combinations thereof. Examples are metal
carboxylates (e.g. iron 2-ethylhexanoate, cobalt 2-ethylhexanoate), metal
alkoxides (e.g. aluminium tri-sec-butoxide, tantalum butoxide), metal
triethanolamines, metal glycolates, organically substituted ammonium
salts of metal-containing complex anions (e.g. anilinium salt of tungstic
acid), metal halides, cyclopentadienyl-containing metal complexes such as
cyclopentadienyl-Co-(CO)2 and/or carbon monoxide containing metal
compounds such as nitroso-cobalt tricarbonyl Co(CO)3NO and others.
The precursors may be diluted with a suitable combustable solvent of low
viscosity or heated in order to reach a viscosity of at most 100 mPas,
preferably at most 40 mPas, more preferably at most 20 mPas. A low
viscosity is regarded advantageously, as it allows an easy spraying of
the fluid. Provided that the viscosity can be reduced to a good sprayable
one, liquid precursors can be directly used, i.e. without additional
solvent.

[0052]Thus, in one embodiment, the combustible liquid precursor solution
only consists of one or more metal comprising compounds.

[0053]In an alternative embodiment, the combustible liquid precursor
solution comprises one or more metal comprising compounds and one or more
solvents. Suitable solvents, which also act as fuels, are known to the
skilled person. The selection of appropriate solvent(s) is inter alia
dependent on its soot forming characteristics in reducing environment and
its dissolution abilities and its viscosity. Suitable viscosity reducing
solvents may comprise one or more acids. While viscosity reducing
solvents may consist of one or more acids, often 50% w/w total acid(s) or
less may be used and in some cases acids are neither needed nor desired.
Preferred solvents include ethers, such as tetrahydrofuran (thf);
aliphatic, linear or branched unsubstituted hydrocarbons, preferably with
1-10 C-atoms such as hexane and carboxylic acids with 1-10 C-atoms such
as 2-ethylhexanoic acid. Particular preference is given to thf due to its
good solvent properties and the low soot formation during combustion.
Aromatic hydrocarbons and long chain aliphatic hydrocarbons proved to be
less suitable, unless specific carbides or carbon layers are desired.

[0054]It was found that some reaction parameters, such as reaction time,
reaction temperature and fuel-oxygen-equivalence, influence the method
according to the invention. Reaction times for the method according to
the invention may vary. Generally, short reaction times, e.g. below 1
second, are preferred. Reaction temperatures for the method according to
the invention may vary and are within the typical range of FSP processes.
Advantageously, the temperature is above 800° C. within a burning
flame (maximal temperature within a specific flame), preferably at least
1000° C., more preferred at least 1300° C. The fuel-oxygen
equivalence is generally above 1; advantageously between 1 and 3.0,
preferably 1.5-3; most preferably between 2.0 and 3.0. It is believed
that this ratio provides a reductive atmosphere allowing formation of the
powders as described above.

[0055]It is believed that the main difference in the present method
compared to the known methods is the incorporation of a flame reactor, in
particular the flame of said reactor, in a protecting/reducing gas
environment thereby. This enables the preparation of reduced compounds
such as metals, non-oxidic ceramics and reduced metal oxides.
Advantageously the whole path, i.e. from the spray nozzle/burner tip to
the particle collector is shielded from ambient air. In order to prevent
any environmental oxygen from coming close to the flame, it is preferred
to shield the flame. This may be accomplished by incorporating it in an
airtight box. Alternatively, this may be accomplished by incorporating
into a cylindrical tube.

[0056]In a specific embodiment of the present invention an additional
porous tube around the flame is applied to influence and stabilize the
production. Through the porous tube additional, optimally reactive gases
as coolant/reacting gas can be applied to influence the gas phase
thermodynamics within the flame. In one embodiment, CO2 is applied.
It is believed that this avoids the production of soot by shifting the
Boudouard equilibrium (CO2+C→2 CO). In another
embodiment, carbon containing gases (such as methane, ethane, acetylene
or propane) are applied. This can be used for the production of metal
carbides and carbon coated materials. In yet another embodiment, nitrogen
containing gases (such as ammonia or nitrogen) are applied. This can be
used for the production of metal nitrides.

[0057]In one embodiment, the energy produced by FSP process as described
herein is uses for a gas/steam turbine.

[0058]In a further embodiment, the FSP process as described herein is also
used for the synthesis of syngas (hydrogen/CO).

[0059]In an additional step it is possible to separate H2 and/or CO
from the off gas and to purify them, e.g. over suitable
adsorption/desorption zeolites.

[0060]In the scope of the present invention it could be shown that the
combustion stochiometry is of importance in order to get the desired
products. It is in particular important that there is not enough oxygen
present for full combustion of the fuel (mainly carboxylic acids,
aromatics and THF). In a preferred embodiment, the process conditions are
such that the fuel combustion product consists mainly of CO and H2
leading to a strongly reducing atmosphere. This allows for the production
of metals and avoids oxidation of the produced metals and avoids soot
production and powder contamination.

[0061]Also of importance is the flame atmosphere. For example at ambient
air concentrations (20% O2) the materials produced by a standard FSP
process are highly stable and fully oxidized ceramic powders with very
high surface areas. When decreasing the oxygen concentration to below
0.1% (volume/volume), novel phases such as reduced metal oxides (e.g.
wustite, molybdenum(IV) oxide) can be produced. At even lower oxygen
concentrations (below 100 ppm) there is not sufficient oxygen present in
the flame for oxidation of the metal to the corresponding oxides or
thermodynamically most favourable oxides, and therefore metallic
particles or non-oxidic ceramics are produced.

[0062]In a preferred embodiment, the combustion of hydrocarbons (more
specifically the C--H bonds present in the solvent or metal comprising
compound) is the major source of energy in the method as described
herein.

[0063]In a further embodiment, the invention relates to a method for the
production of metals or alloys, such as cobalt, iron, copper, nickel, in
particular fcc cobalt, wherein said method comprises flame spray
pyrolysis (FSP) of one or more precursor solution containing combustible
metal(s), and wherein said method being performed in an atmosphere with
an O2 concentration below 100 ppm (volume/volume) measured in the
off-gas and wherein said method being performed using an
oxygen-containing gas as oxidizing agent and wherein the obtained powder
is optionally subjected to a sintering step with a sintering temperature
of at least 40%, preferably 50-70% of the metal melting point. The thus
obtained nanocrystalline metal materials are novel and subject to the
present invention. Such nanocrystalline metals may be used as coating of
wear-resistant materials and high-wear tools.

[0064]In a further embodiment the invention relates to a method as
described herein wherein one or more metal comprising compounds selected
from the group of Cu, Ag, Fe, Co, Mo, Bi, W, preferably Cu, are used and
wherein the combustible solvent is selected from aliphatic or cyclic
compounds, avoiding aromatics and long aliphatic chains (>C8), and are
preferably selected from tetrahydrofurane (THF), short carboxylic acids
(e.g. 2-ethylcarboxyc acid) and the metal salts of short carboxylic acids
(e.g. copper 2-ethylhexanoate) and wherein the fuel-oxygen equivalence
ratio is between 1.0-2.0. As analysis confirms, carbon-coated metal
powders are obtained. The thus obtained powders are novel and also
subject to the present invention. After compaction, such powders have a
negative temperature coefficient linearized between 30 and 100° C.
of at least 3500 K, more preferably at least 4000 K and most preferably
at least 4500 K. Further, such powders may be used as sensor material in
temperature and/or pressure sensitive materials.

[0065]In a further embodiment, the invention relates to a method for the
production of metal/ceramic nanocomposites, wherein said method comprises
flame spray pyrolysis (FSP) of a combustible solution comprising at least
two metals; at least one which can be reduced in the flame (reduction
potential above -0.42 V as defined by Riedel) such as e.g. bismuth,
cobalt, copper, iron, silver, gold, nickel and others and mixtures
thereof and at least one metal which can not be reduced in the flame
(standard potential below -1.00 V as defined by Riedel) such as e.g.
cerium, titanium, silicon, zirconium, yttrium, magnesium, aluminium and
others and mixtures thereof and wherein said method being performed in an
atmosphere with an O2 concentration below 100 ppm (volume/volume)
measured in the off-gas and wherein said method being performed using an
oxygen-containing gas as oxidizing agent and wherein N2 is applied
to the flame. The thus obtained metal/ceramic nanocomposites are novel
and subject to the present invention. Preferred metal/ceramic
nanocomposites are bismuth/ceria nanocomposites. The metal/ceramic
composites consist of ceramic and metal particles, where the number
averaged primary particle size of the ceramic particles and the number
averaged primary particle size of the metal particles are both below 300
nm and both individual particle size distributions evaluated from
transmission electron microscopy images have geometric standard
deviations of below 1.6, more preferably below 1.4. After compaction
and/or heat treatment to temperatures below the melting point of the
metal, the composites show metallic conductivity and reflectivity and a
Vickers hardness of at least twice, more preferably at least 4 times and
most preferably at least 7 times the Vickers hardness of the pure
coarse-grained bulk metal. Such metal/ceramic nanocomposites may be used
in electrical elements for an enhanced thermoelectric effect or in metals
and alloys with enhanced mechanical, optical, magnetic, electronic or
chemical properties.

[0066]In a further embodiment, the invention relates to a method for the
production of metal powders, non-oxidic ceramic powders and reduced metal
oxide powders as well as mixtures and alloys thereof, wherein the powders
are synthesized by flame spray pyrolysis (FSP) of a combustible precursor
solution, said method being performed in an atmosphere with an O2
concentration below 1000 ppm (volume/volume) measured in the off-gas.

[0067]In a second aspect, the invention relates to an specifically adapted
apparatus for FSP synthesis ("FSP apparatus").

[0068]FSP apparatus are known and usually comprise one or more burner(s),
devices for supplying the burner(s) (e.g. liquid pumps, gas feeding) and
devices for collecting the obtained powder (e.g. a filter). Such devices
are known in the field. According to this invention, the flame of said
burner is shielded from environmental air. This may be accomplished by
use of an airtight box or by use of a cylindrical tube.

[0069]Thus, in one embodiment, the invention relates to an FSP apparatus
wherein burner, flame and collecting device are located in a (completely)
closed room or container filled with an inert or reducing gas (such as
N2, Ar, He, H2, CO and mixtures thereof).

[0070]In an alternative embodiment, the flame is shielded from
environmental air by using a porous tube, e.g. a porous sinter-metal
porous graphite or a porous silicium-carbide tube preferably a porous
sinter-metal tube.

[0071]In an alternative embodiment, the flame is shielded from
environmental air by using a non-porous ("tight") tube. In some cases,
this set-up results in thermophoretically driven particle deposition,
powder loss and inhomogenity and is therefore less preferred.

[0072]In an advantageous embodiment, the flame is shielded from
environmental air by using a tube comprising a porous inner tube and
distant there from a tight outer tube. Such a setup is shown in FIG. 3.
Using such a tube additional inert and/or reactive gases may be applied
to the flame as described below. This set-up allows the continuous
production of non-oxidic ceramic powders or reduced metal oxide powders
of high quality by providing an aerodynamically designed tubular reactor.
It is believed that the radial inflow of coolant/reactant gases avoids
the deposition of any powder on the inner wall of the tube.

[0073]In a further embodiment, the invention relates to an FSP apparatus
wherein the flame is shielded from environmental air by a device which
also allows radial application of gases. Such device preferably comprises
a porous cylindrical tube. Such device also gives the possibility of
introducing other gases to the flame atmosphere.

[0074]Such gases may be used to shift the Boudouard equilibrium and
therefore control the formation or amount of soot on or in the product.

[0075]Further, such gases may be reactive gases (CO2, CO, H2,
NH3, CH4, H2S, etc.) as well as metal and semi-metal
containing gases (e.g. H2Se, H2Te, B2H6, SiH4,
(CH3)SiH3, (CH3)2SiH2 (CH3)3SiH,
(CH3)4Si) and organic compounds and monomers (butadiene,
ethylene, ethane, acetylene, propane, etc). Such gases influence the
properties of the flame and/or of the powders obtained. For example,
monomers may lead to polymer layers protecting the powders, other
compounds may lead to coatings altering the electronic, magnetic or
chemical properties of the powders.

[0076]Further, previously evaporated liquids such as liquids selected from
the group comprising of acrylonitril, siloxanes (e.g.
hexamethyldisiloxane, hexamethyldisilazane, tetramethoxysilan,
tetraethoxysilan), titanium-tetraisopropoxide and mixtures thereof may be
applied to the flame. Such liquids influence the properties of the flame
and/or of the powders obtained. For example, the formation of polymer- or
ceramic coated particles is possible. Such coatings protect the powders
obtained or alter the electronic, magnetic or chemical properties of the
powders obtained.

[0077]Also encompassed in the invention are mixtures of the above
mentioned gases/liquids and devices allowing the application of such
mixtures.

[0078]The "porous tube" referred to in this specification may be made of
any material which is resistant to the reaction conditions and inert
towards the starting materials/produced powders. Examples are sintered,
porous steel and porous ceramic material, preferably silicium carbide.
Further, said porous tube has an inner diameter of at least 2 cm and a
length of at least 5 cm as well as a ratio between the height and the
inner diameter of at least 2.0.

[0079]In an advantageous embodiment, the FSP apparatus as described herein
is connected to a gas/steam turbine wherein the off-gases of said FSP
apparatus are used to operate said gas/steam turbine. The combination of
an FSP apparatus and a gas/steam turbine allows efficient use of the off
gasses for energy production. Such gas/steam turbines are known, and may
be adapted to the size of the FSP apparatus. Alternatively, the FSP
off-gasses are only part of the gasses feed into the gas/steam turbine.

[0080]In a third aspect, the invention relates to new materials obtainable
by a process as described herein. Such materials exhibit specific new or
improved properties. Such materials include un-agglomerated, air-stable
copper or cobalt powder of narrow size distribution, metals coated with
carbon, metal/ceramic composites (Such as Bi/CeO2 composites).

[0081]A first group of compounds obtainable according to a method of this
invention are metal-oxides with oxygen content lower than the oxygen
content that is considered as thermodynamically stable at room
temperature in air. Examples are reduced titanium(III-IV)oxides,
cerium(III-IV)oxides, iron(II) oxide, cobalt(II) oxide, molybdenum(IV)
oxide, tungsten(IV) oxide. Thus, the inventive method allows the
production of reduced metal oxides, e.g. TiO2-x, FeO, MoO2,
WO2, CoO, COAlOx in high phase purity. Such materials have e.g.
applications in semiconductor technology, as solid-oxide fuel cell
materials, as diode materials, as switches and sensor and as pigments.

[0082]A further group of compounds obtainable according to a method of
this invention are metal powders which are coated by a protecting layer
of elements and compounds of elements of the non-metals of the second or
third period of the periodic table, especially carbon and silicon
containing compounds.

[0083]A further group of compounds obtainable according to a method of
this invention are metals and metal alloys or composite oxide/metal
compounds with a standard potential between +0.52 eV and -0.41 eV as
defined in Riedel 1999.

[0084]With the method of the present invention metallic powders can be
produced that gain at least 70%, preferably at least 90%, most preferred
at least 98% of the weight which corresponds to a full oxidation of the
pure metal to the corresponding thermodynamically stable oxide upon a
treatment consisting of a 1 hour oxidation in ambient air at 800°
C.

[0085]Particles that are obtainable by the method of the present invention
are e.g. metallic particles of a metal or an alloy containing a metal
with a standard potential between +0.52 eV and -0.80 eV, preferably +0.15
eV and -0.80 eV, most preferred -0.13 eV and -0.80 eV (all as defined by
Riedel).

[0086]While it is possible without specific provisions to prepare
particles with down to -0.42 eV, specific care must be taken to keep the
O2 level low if a metal with a potential lower than -0.42 eV shall
be processed. Thus, in view of procedural economy, metals with standard
potential between +0.52 eV and -0.42 eV, preferably +0.15 eV and -0.42
eV, especially -0.13 eV and -0.42 eV (all as defined by Riedel) are
preferably used in the inventive method. The method is especially
applicable to metals with a redox potential >Fe, e.g. Co, Cu, Bi, W,
Mo, Ni, Pb, as well as mixtures and alloys thereof. Such materials have
e.g. applications in hard and soft magnetic materials, as electric
conductivity and temperature conductivity increasing materials in
liquids, polymers and ceramics, as building blocks for wires, switches
and sensors as well as for uses in powder metallurgy.

[0087]A further group of compounds obtainable according to a method of
this invention are non-oxidic ceramic powders. In a preferred embodiment
of the present invention, the inventive method is used for obtaining
non-oxidic ceramic powders containing a transition metal, especially a
transition metal selected from the group comprising tungsten, molybdenum,
cobalt, nickel, chromium and mixtures thereof, and non-metallic elements
from the group comprising boron, carbon, nitrogen, phosphorus, sulphur,
silicon, arsenic, antimony, germanium and mixtures thereof. Non-oxidic
ceramic particles obtainable by the inventive method are in particular
transition metal carbides, nitrides, sulfides, silicides, selenides,
tellurides, borides.

[0088]Inventive particles of tungsten carbide (WC or W2C) and
metallic copper can be used as materials in high voltage contacts, and WC
or W2C is suitable as platinum catalyst substitute, e.g. in
hydrogenation reactions.

[0089]A further group of compounds according to this invention encompasses
mixtures, alloys and composites of materials directly obtainable from the
above method and mixtures and composites of materials obtainable with the
inventive method and any metal-oxide. Thus, particles further obtainable
by the inventive method are composites of materials as defined above as
well as metal oxides, polymers and carbon, including particles that are
composites in a core-shell type structure. In one embodiment of the
inventive method nanoparticles are obtainable that are metallic particles
covered with an inert layer, in particular with a thin carbon layer.

[0090]Also preferred embodiments of the present invention are alloys
containing among others, bismuth, silver and copper.

[0091]It is also possible to directly produce mixed materials, e.g.
metallic iron and ZrO2 or metallic bismuth and CeO2 etc., i.e.
a mixture of metal particles and ceramic particles. By such a procedure,
the material characteristic can be well directed, e.g. predominantly
ceramic or predominantly metallic.

[0092]The volume-surface-average diameter (as defined in Janssen 1994) of
the particles obtainable with the inventive method is lower than 300 nm,
preferably below 100 nm, much preferred below 50 nm. The
volume-surface-average diameter, dp, can be calculated from the
specific surface area assuming spherical particles as
dp=6/(ρ*SA), where ρ is the density of the material
(kg/m3) and SA is the specific surface area (m2/kg). The method
of the present invention allows producing powders with volume-surface
average diameters in the nano-scale, in particular below 300 nm. Thus,
the invention provides nanoparticles having a volume-surface-average
diameter as defined in Janssen 1994 below 300 nm, preferably below 100
nm, much preferred below 50 nm.

[0093]In addition, the nano-powders of the present invention in general
have very narrow size distributions measured as geometric standard
deviation σg according to Grass and Stark (2005). The
hydrodynamic particle size distribution is measured from a stable colloid
suspension using an X-ray disc centrifuge such as a BI-XDC from
Brookhaven Instruments. The geometric standard deviation is calculated by
fitting a log-normal distribution to the measured data using the
least-squares method. The geometric standard deviation σg
preferably is below 1.6, much preferred below 1.4. Without being bound to
theory, this narrow size distribution may be attributed to a turbulent
flame process (Vemury and Pratsinis 1995, Grass and Stark 2005).
Depending on the sintering characteristics of the material the produced
powders can be spherical, e.g. in the case of cobalt, or strongly
agglomerated, e.g. in the case of copper and bismuth.

[0094]By the method of the present invention several different materials
can be produced and even the material characteristics controlled and
adapted. It is e.g. possible to produce metal particles that have a
"clean", metallic surface or that are covered by e.g. a carbon layer.
Specific materials and composites as well as some of their uses are
identified below. These materials are new and encompassed by the present
invention:

[0095](1) Carbon coated copper: A carbon layer protects the metal from
oxidation at ambient temperature and from sintering. If e.g. one form of
the carbon coated copper (Cu/C) is formed into a pill, its conductivity
is temperature and pressure dependent, thus making it a highly
interesting material for applications in pressure/temperature sensors. If
treated with e.g. HNO3, the copper may be removed, leaving a light
weight carbon product with open pores. Another form of the inventive Cu/C
can be dispersed in ethanol resulting in an ink suitable for the
production of printed circuits, namely in that after application and
drying the carbon is removed, e.g. by a high temperature procedure under
CO2, and the copper then is e.g. laser sintered.

[0096](2) Copper (without carbon): Cu can be used to enhance the
conductivity (heat and electricity) of polymers and liquids such as e.g.
ethylene glycol or silicon oils in heat exchangers or in contact pastes.

[0097](3) Metallic Bi nanoparticles alloys and composites, optionally with
an oxide compound such as ZrO2 or CeO2 as dopant, can be used
as photomascs, in that the originally opaque layer is laser sintered to
give a transparent layer in desired regions as well as in thermo-electric
materials.

[0098](4) The direct production of steel is also an application of the
method of the present invention, whereby the alloying metals, e.g. Co, Ni
etc. are injected together with the iron, preferably dissolved in the
same precursor solution, leading to a product with very small grains.

[0099](5) Low melting materials, such as Bi, Cu and Ag alloys may be used
for soldering in electronics or Babbitt bearings, since due to the small
particle size the product is much better melting and lubricating.

[0100](6) Oxide dispersed steel (ODS), i.e. steels of different
composition, may be manufactured using the method of this invention. Due
to about 1% Y2O3 such materials have a high load resistance.

[0101]In a further aspect, the present invention relates to the use of the
powders obtained by a method as described herein.

[0102]The powders obtained by a process as described herein may be used in
a wide range of technical applications. These uses include applications
for powder metallurgy, as materials with enhanced electronic, magnetic
and/or mechanical properties, especially for use as conductivity
increasing materials in fluids, polymers or ceramics, as hard or soft
magnetic materials for magnets or transformers, as building blocks for
wires, sensor or switches in the electronic industry and as raw product
for powder metallurgy in the machining industry and as low melting alloys
for interconnects in electronics. Further uses of powders of the present
invention are as reactive surfaces, as ceramics, building materials and
in heterogeneous catalysis.

[0103]Specific uses of the materials obtainable according to a method as
described herein are identified below:

[0104](1) Reduced metal oxide powders of the present invention can be used
in pigments for paints, inks or in cosmetic applications, or in
semi-conducting materials, or in sensors, or in diodes in electronic
applications or in solid state fuel cells.

[0105](2) Non-oxidic ceramic powders of the present invention, in
particular tungsten-carbide, cobalt nitride, molybdenum carbide,
molybdenum, tungsten, can be used in high duty applications such as
cutting and drilling tools and protective coatings.

[0106](3) Metal powders of the present invention, in particular iron,
steel, copper, cobalt and tungsten can be used as materials with enhanced
electronic, magnetic and/or mechanical properties, especially for use as
conductivity increasing materials in fluids, polymers or ceramics, or for
use as hard or soft magnetic materials for magnets or transformers or
coil cores, or for use as building blocks for wires, sensor or switches
in the electronic industry and as raw product for powder metallurgy in
the machining industry and as low melting alloys for interconnects in
electronics.

[0107](4) Alloys containing among others bismuth, silver and copper,
especially low melting alloys, may be used as interconnects for
electronics.

[0108](5) Materials obtainable by the method of the present invention may
be used in catalysis, including platinum-like catalysis and
hydrodenitrogenation. A specific use for tungsten-carbide is in
platinum-like catalysis such as hydrogenation and metal nitrides as well
as alloys of metal nitrides for hydrodenitrogenation.

[0109](6) Metal ceramic composites can be used for materials with
increased mechanical properties and sintered nanocrystalline bulk
materials with increased mechanical properties for use as wear resistant
materials and coatings in cutting, drilling and machining applications.

[0110](7) Fcc Cobalt manufactured by a method as described herein may be
used for sinter resistant, high wear materials/coatings.

[0111](8) Particles of Co, Fe and other metals/alloys with similar
magnetic features coated with carbon manufactured by a method as
described herein may be used in combination with polymers for the
preparation of polymer based magnets for use in electronics as e.g.
electronic motors or generators and for the preparation of magnetic glues

[0112](9) Bi/ceramic nanocomposites manufactured by a method as described
herein may be used for manufacturing of thermoelectric elements.

[0113](10) Co, Fe and other metals/alloys with similar features may be
used in electronic/magnetic applications such as re-writable data
carriers, e.g. hard discs.

[0114](11) Co, Fe and other metals/alloys with similar features coated by
carbon layers may be used for separation techniques.

[0115](12) Alloys containing amongst others lead, copper, tin, antimony
and bismuth for may be used for the fabrication of Babbitt bearings.

EXAMPLES

[0116]The invention is further described by way of examples. All examples
consist of 3 steps;

[0123]Electronic resistance/conductivity: Pills with a diameter of 1.3 cm
were pressed from selected examples using a uniaxial press at 370 MPa.
The electronic resistance of the pills was measured using a multimeter
(Voltcraft VC 220). The temperature dependence of the electrical
conductivity was measured in a glove box in an oxygen free atmosphere.
The method applied is presented in detail in Changyi et al. 2000.

[0124]General Experimental Procedure and Set-up: Gas concentrations are
given in % volume per volume of gas unless otherwise stated. Reduced
powders were produced by flame spray pyrolysis in a laboratory scale
setup consisting of an air-assisted nozzle 21 as described in detail by
Madler et al. (2002), (e.g. FIG. 1 of said document). Such a nozzle was
placed in a glove box 24 in which the atmosphere could be controlled and
measured by a mass spectrometer (see FIG. 2). Metal containing liquids
(precursor liquids, precursor solutions) were brought into the flame by a
micro-gear-ring pump (HNP Mikrosysteme GmbH) at 3-7 ml/min. The flame
consisted of a central spray delivery, a premixed circular support flame
(diameter 6 mm slit width 150 μm, 2.2 l/min oxygen (PanGas, tech), 1.2
l/min methane (PanGas, tech)) and a circular sheath gas delivery. Oxygen
(PanGas, tech) as well as methane (PanGas, tech) at different flow rates
were delivered as dispersion gas in all experiments and delivered over a
nozzle pressure drop of 1.5 bar. All gas flow rates were controlled by
calibrated mass flow controllers (Brooks 5850S). The process conditions
for all experiments are outlined in Table 1. The off gas was freed from
the powder by a glass fiber filter 22 and piped into a series of two
columns 23 filled with zeolite 4A and 13X, respectively, for the removal
of water and carbon dioxide. For examples 2, 4, 6, 10, 11, 12, 14 and 15
a porous sinter metal tube was used for better control of the flame
conditions. The construction of the tube is outlined in FIG. 3a together
with the tube cross section at height 10 shown on the right FIG. 3b. A
sinter-metal tube 1 (GKN Sintermetalle, 9366, inner diameter 25 mm, wall
thickness 6 mm, Material: 1.4404 R 5 IS, length: 150 mm) was embedded in
an aluminium tube 6, closing both ends between the two tubes 7. Cold Gas
was introduced by a port 2, flowed through the sinter-metal tube 9,
cooled the flame 5 and flowed towards the hood 8. The tube was placed
directly on the burner head 4.

[0125]Precursors: Metal containing liquid precursors were produced
according to the following methods.

[0126]Copper precursor I: 90 g Copper-acetat (Fluka, 99%) were dissolved
in 500 ml 2-ethylhexanoic acid during 2 hours at 140° C. and water
and acetic acid were removed by distillation. The resulting liquid was
diluted with THF to give 750 ml of a precursor with a suitably low
viscosity.

[0127]Copper precursor II: 90 g Copper-acetat (Fluka, 99%) were dissolved
in 500 ml 2-ethylhexanoic acid during 2 hours at 140° C. and water
and acetic acid were removed by distillation. The resulting liquid was
diluted with xylene to give 1000 ml of a precursor with a suitably low
viscosity.

[0128]Cobalt precursor: 50 g Cobalt-acetat tetrahydrat (Fluka 99% Lot Nr
050392-BS) were dissolved in 280 ml 2-ethylhexanoic acid during 2 hours
at 140° C. and water and acetic aced were removed by distillation.
The resulting liquid was diluted with THF to give 500 g of a precursor
with a suitably low viscosity.

[0134]For reference, 30 ml of the molybdenum precursor were flame sprayed
in an air atmosphere (process conditions see Table 1) according to patent
application WO2004/103900 giving a greenish powder. XRD confirmed
formation of molybdenum(VI) oxide (see FIG. 4,A) with a specific surface
area of 75 m2/g.

[0135]Reference Material Preparation 2: Cobalt(II,III)oxide Powder

[0136]For reference, 30 ml of the cobalt precursor were flame sprayed in
an air atmosphere (conditions see Table 1) according to patent
application WO2004/103900 giving a black powder. XRD confirms formation
of cobalt(II,III)oxide (FIG. 5,A) with a specific surface area of 86
m2/g.

[0138](MoO2--Mo composites): 30 ml of the molybdenum precursor were flame
sprayed in a nitrogen atmosphere with an oxygen concentration of 100 ppm
(conditions see Table 1). XRD confirmed the formation of bluish
molybdenum(IV) oxide together with smaller amounts of metallic molybdenum
(see FIG. 4,C). Upon oxidation in air at 500° C. the powders mass
increased by 10% (theoretical Mo→MoO3: 50%) and further
confirmed the presence of reduced molybdenum oxides and metallic
molybdenum in the as prepared sample. After oxidation the colour turned
green.

[0140](Cobalt nano powder): 30 ml of the cobalt precursor were flame
sprayed in a nitrogen atmosphere with an oxygen concentration of less
than 100 ppm (conditions see Table 1). The sinter metal tube was used for
cooling and reacting of the flame with CO2 (30 l/min, PanGas 4.0).
XRD confirmed the formation of metallic cobalt in its face-centered-cubic
(fcc) phase (see FIG. 5C). The XRD pattern further shows small peaks
which were attributed to cobalt nitride that acts as protecting layer on
the surface of the particles. This is further supported by the fact, that
the powder proved to be fully inert in air at room temperature. Upon
oxidation at 500° C. in air during 1 h the powder mass increased
by 33% (theoretical Co→CO3O4: 36%). The powder with a
specific surface area of 14.3 m2/g had a very narrow size
distribution with a geometric standard deviation σg of below
1.6 as measured according to Grass and Stark 2005. The powder further
consisted of spherical non-agglomerated particles further illustrated by
the transmission electron micrograph in FIG. 6. When pressed to a pill
the powder exhibited a low electronic resistance (below 0.1 Ohm m),
strong superparamagentic properties as well as metallic gloss. When the
powder was ignited with a flame the powder burned rapidly forming cobalt
oxide.

[0143](Iron nano powder): 30 ml of the iron precursor were flame sprayed
in a nitrogen atmosphere with an oxygen concentration of below 100 ppm
(conditions see Table 1). The sinter metal tube was used for cooling of
the flame with N2 (30 l/min, PanGas 5.0). XRD confirmed the
formation of pure metallic iron (FIG. 8,B). Upon oxidation in air at room
temperature the powder weight increased by 2.6% within 5 minutes and
remained stable for at least an hour. At 400° C. the powder weight
increased by further 25.7% (theoretical Fe→Fe2O3: 43%).
When the powder was ignited with a flame it burned rapidly forming red
iron oxide.

Example 7

[0144](Tungsten-Tungsten oxide composites): 30 ml of the tungsten
precursor were flame sprayed in a nitrogen atmosphere (conditions see
Table 1). The produced powder was a composite of metallic tungsten and
tungsten(VI) oxide as could be seen by the XRD pattern as shown in FIG.
9.

Example 8

[0145](Copper-carbon composites): 30 ml of the copper precursor 2 were
flame sprayed in a nitrogen atmosphere (conditions see Table 1). FIG. 11
shows a transmission electron micrograph of the as produced powder. It is
clearly visible, that very small copper particles (below 10 nm) were
embedded in a graphite matrix. The powder specific surface area was 196
m2/g.

Example 9

[0146](Copper embedded in carbon nano-containers): 30 ml of the copper
precursor 1 were flame sprayed in a nitrogen atmosphere (conditions see
Table 1). The powder could be characterized as pure copper by
XRD-analysis (FIG. 10). Transmission electron microscopy (FIG. 12) showed
that the copper particles were coated by a very thin layer of carbon
about 0.5-1 nm thick. This carbon layer was found to give the powder an
excellent stability in air and even stability against diluted nitric acid
as shown in FIG. 13. When pressed to a pill the black powder exhibited a
strongly temperature and pressure dependent electronic resistance with a
negative temperature coefficient linearized between 30° C. and
100° C. of >4500 K (FIG. 14).

Example 10

[0147](Copper nano powder): 30 ml of the copper precursor 1 were flame
sprayed in a nitrogen atmosphere with an oxygen concentration of 100 ppm
(conditions see Table 1). The sinter metal tube was used for cooling of
the flame with nitrogen (30 l/min, PanGas 5.0). The resulting powder
consisted of pure copper metal: When pressed to a pill (using an uniaxial
press at 370 MPa) the powder exhibited an extremely low electronic
resistance (less than 0.03 Ohm m) as well as a strong metallic gloss.
Upon ignition with a flame the powder burned giving black copper oxide.

Example 11

[0148](Bismuth nano powder): 30 ml of the bismuth precursor were flame
sprayed in a nitrogen atmosphere (conditions see Table 1). The sinter
metal tube was used for cooling of the flame with nitrogen (30 l/min,
PanGas 5.0). The XRD pattern of the black powder showed the formation of
pure metallic bismuth (FIG. 15). The morphology of the powder was shown
by a scanning electron micrograph as presented in FIG. 16. Upon ignition
with a flame the powder oxidised to yellow bismuth oxide.

Example 12

[0149](Iron-Cobalt alloy nano powder): 28.5 g of the iron precursor were
mixed with 28.5 g of the cobalt precursor and sprayed in a nitrogen
atmosphere (conditions see Table 1). The sinter metal tube was used for
cooling of the flame with nitrogen (30 l/min, PanGas 5.0). A small
section of the XRD pattern of the as prepared powder is shown in FIG. 16
B. For reference the XRD pattern of example 6 is shown below (FIG. 16 A).
It is clearly visible, that the XRD-peak at 2-Theta ˜44.3°
from alpha-iron is shifted towards larger 2-Theta values. This gives
clear evidence that the cobalt (24% wt) is solubized in the iron matrix
(76% wt) forming an alloy known as Permendur 24. In FIG. 17, the breath
of the peaks as well as the SEM micrograph show that the produced grains
are of nano-scale dimensions.

[0151](Carbon coated cobalt powder): 30 ml of the cobalt precursor were
flame sprayed in a nitrogen atmosphere (conditions see Table 1). The
sinter metal tube was used for cooling of the flame with a mixture of
nitrogen (30 l/min, Pan Gas 5.0) and acetylene (5 l/min, PanGas, tech).
The XRD pattern of the powder showed the formation of metallic cobalt.
Several graphene layers coating the individual cobalt nanoparticles could
be observed by transmission electron microscopy (FIG. 20) and resulted in
a carbon content as measured by microanalysis (LECO 900) of 2.2% wt.

Example 15

[0152](Bismuth/ceria nanocomposites): 30 grams of bismuth precursor were
mixed with 4.6 grams of ceria-octoate (Shephard, 12% wt Cerium) and flame
sprayed in a nitrogen atmosphere (conditions see Table 1). The sinter
metal tube was used for cooling of the flame with nitrogen (30 l/min,
PanGas 5.0). The resulting powder consisted of a composite of ceria and
bismuth nanoparticles as shown by X-ray diffraction (FIG. 21). When
pressed to a pill at 370 MPa (FIG. 22, uniaxially applied pressure) the
conductive (>100 S m-1) material of >90% relative density had
a Vickers hardness of HV=120 therefore strongly exceeding the Vickers
hardness of pure bismuth (HV=16).

[0154]In addition to the data provided above, the following should be
noted:

[0155]Particle size: All powders had dimensions in the nano-scale whereby
at least 99% of the particles (by number) had dimensions below 200 nm.
This can be either seen from TEM and SEM images (FIGS. 6, 11 and 15) as
well as from the breadth of the peaks in the XRD patterns which can be
related to the crystallite size using the Scherrer Formula.

[0156]Particle size distributions: All powders had very narrow size
distributions as measured by Grass and Stark (2005) (FIGS. 6, 11 and 15)
and as further expected from a turbulent flame process (Vemury and
Pratsinis 1995, Grass and Stark 2005). Said distribution can be
characterized by a geometric standard deviation σg of smaller
than 1.6, preferably smaller than 1.4. In materials consisting of more
than one material (e.g. metal/ceramic nanocomposites) the particle size
distribution of each individual material present in the produced powder
can be characterized by a geometric standard deviation σg of
smaller than 1.6, preferably smaller than 1.4.

[0157]Shape: Depending on the sintering characteristics of the material
the produced powders were either spherical or slightly agglomerated
forming fractal agglomerates consisting of 2-20 primary particles each.

[0158]Combustion stochiometry: For all examples presented above not enough
oxygen was present for full combustion of the fuel (mainly carboxylic
acids, aromatics and THF). The process conditions were optimized so that
the fuel combustion product consisted mainly of CO and H2 leading to
a strongly reducing atmosphere. This allowed for the production of e.g.
metals. In addition, these conditions allowed to avoid oxidation of the
produced metals as well as soot production and powder contamination.

[0159]Flame atmosphere: Examples 1-2 for molybdenum, 3-4 for cobalt and
7-8 for iron showed the large influence of the oxygen concentration in
the atmosphere surrounding the flame. At ambient air concentrations (20%
O2) the produced materials were highly stable and fully oxydized
ceramic powders with very high surface areas. When decreasing the oxygen
concentration to below 0.1% (volume/volume), novel phases such as reduced
metal oxides (wustite, molybdenum(IV) oxide) could be produced. At even
lower oxygen concentrations (below 100 ppm) there was not sufficient
oxygen present in the flame for oxidation of the metal to the
corresponding oxides or thermodynamically most favourable oxides, and
therefore metallic particles or non-oxidic ceramics were produced. It was
found that the flame atmosphere could be very well controlled by using an
additional sinter-metal tube. This setup allowed the continuous
production of non-oxidic particles or reduced oxides of high quality by
providing an aerodynamically designed tubular reactor. It was found that
by the radial inflow of coolant/reactant gases the deposition of any
powder on the inner wall of the tube can be avoided. The use of
non-porous tubes resulted in thermophoretically driven particle
deposition and powder loss and inhomogenity. The above described porous
tube also gave the possibility of introducing other gases to the flame
atmosphere as seen in Example 6 where CO2 was introduced to shift
the Boudouard equilibrium and therefore control the formation or amount
of soot on or in the product.

[0160]Starting materials: If several reducible metals are used in the
process as described herein, alloys or nanocomposites containing the
corresponding metals can be manufactured (see examples 12 and 15).

[0181]While there are shown and described presently preferred embodiments
of the invention, it is to be distinctly understood that the invention is
not limited thereto but may be otherwise variously embodied and practiced
within the scope of the following claims.